The invention concerns a method for partially regenerating the catalytic properties of an aged maleic anhydride catalyst. In particular, it has been found that the catalytic properties of a vanadium-phosphorus-oxygen oxidation catalyst used in the conversion of hydrocarbons to maleic anhydride can be partially regenerated by contacting the aged catalyst with sulfur trioxide.
Maleic anhydride is a commercially valuable chemical. It can be used alone or with other acids in the manufacture of alkyd and polyester resins. It is a versatile chemical intermediate also useful as a monomer to produce various copolymers, such as the copolymer of maleic anhydride and vinyl acetate. Significant quantities of maleic anhydride are produced each year to satisfy these needs.
The prior art teaches that maleic anhydride can be produced by oxidizing hydrocarbons, such as butane, butene, butadiene and benzene. The oxidation is carried out in the presence of an oxidation catalyst. The prior art further teaches that vanadium-phosphorus-oxygen catalysts are especially active to catalyze the vapor-phase oxidation of hydrocarbons to maleic anhydride. For instance, the high-surface-area catalyst described in U.S. Pat. No. 3,864,280, granted to Schneider on Feb. 4, 1975, provides high yields of maleic anhydride from butane. These catalysts comprise a vanadium-phosphorus-oxygen complex having an intrinsic surface area in the range from about 7 to 50 square meters per gram, a phosphorus to vanadium atomic ratio in the range of 0.9-1.8:1, and an average vanadium valence in the range 3.9 to 4.6.
During the oxidation process, the catalyst is contacted with hydrocarbon under reducing conditions. Typically the oxidation is carried out at from about 300.degree. C. to about 500.degree. C. In practice, an oxidation feed gas is fed to a tube reactor containing the catalyst material. Shortly into the reactor the hydrocarbon begins to oxidize rapidly thus creating a hot-spot. It has been observed that as the time of reaction on the catalyst increases, the activity and selectivity of the catalyst decreases. This is called "catalyst aging."
Accordingly, it is desirable to find a process for regenerating catalytic properties of aged catalyst to regain the initial high catalyst activity and selectivity. The selection of a suitable regenerating agent and the regeneration conditions which are used are critical, since over-regeneration of the catalyst has been found to result in a very non-selective catalyst. U.S. Pat. No. 4,020,174, issued to Partenheimer on Apr. 26, 1977, describes the use of regenerating agents to reactivate the catalytic properties of vanadium-phosphorus-oxygen catalysts. The agents are selected from the halide-containing compounds.